Conventionally, there has been proposed a display device with a photosensor that, due to including a photodetection element such as a photodiode inside a pixel, can detect the brightness of external light and pick up an image of an object that has come close to the display. Such a display device with a photosensor is envisioned to be used as a bidirectional communication display device or display device with a touch panel function.
In a conventional display device with a photosensor, when using a semiconductor process to form known constituent elements such as signal lines, scan lines, TFTs (Thin Film Transistors), and pixel electrodes on an active matrix substrate, photodiodes or the like are formed at the same time on the active matrix substrate (see PTL 1).
FIG. 15 shows an example of a conventional photosensor (PTL 2 and 3) formed on an active matrix substrate. The conventional photosensor shown in FIG. 15 is configured by a photodiode D1, a capacitor C2, and a transistor M2. The anode of the photodiode D1 is connected to wiring for supplying a reset signal RS. The cathode of the photodiode D1 is connected to one of the electrodes of the capacitor C2 and the gate of the transistor M2. The drain of the transistor M2 is connected to wiring for supplying a constant voltage Vsup. Note that sensor circuit output data SData of the photosensor is output from the source of the transistor M2. The other electrode of the capacitor C2 is connected to wiring for supplying a readout signal RW
In this configuration, sensor circuit output data SData that is in correspondence with the amount of light received by the photodiode D1 can be obtained by supplying the reset signal RS and the readout signal. RW in accordance with respective predetermined timing. Next is a description of the operation of the conventional photosensor shown in FIG. 15, with reference to FIG. 16. Note that in FIG. 16, the low level (e.g., −4 V) of the reset signal RS is indicated as −Vb, and the high level (e.g., 8 V) of the readout signal RW is indicated as Vrw. It should also be noted that the high level of the reset signal RST is considered to be 0 V, and the low level of the readout signal RW is considered to be 0 V.
A sensing sequence of the photosensor shown in FIG. 15 is described below in three parts, namely (A) readout period, (B) reset period, and (C) sensing period.
(A) Readout period
The readout period corresponds to the period in which the readout signal RW is at the high level. While the readout signal RW is at the high level, the potential VINT of a storage node is “pulled up” via the capacitor C2. Note that the storage node is a connection point between the capacitor C2, the cathode of the photodiode D1, and the gate of the transistor M2. At this time, letting the storage node potential immediately before readout be V0, the capacitance of the capacitor C2 be Cst, the total in-circuit capacitance be Ctotal, and the amplitude of the readout signal RW be Vrw, the potential VINT of the storage node can be obtained by the below expression.VINT=Cst/Ctotal·Vrw+V0 
Then, due to the “pulled-up” potential VINT exceeding the threshold value of the transistor M2, the transistor M2 is turned on, and the sensor data SData is output. At this time, letting the threshold value of the transistor M2 be Vth, the current value of a constant current source be I, and the conductance of the transistor M2 be β, the potential Vout of SData can be obtained by the below expression.Vout≈VINT−Vth−(2I/β)1/2 
(B) Reset period
Due to the reset signal RS switching to the high level (0 V), a forward current flows to the photodiode D1, and the potential VINT of the storage node is reset to 0 V.
(C) Sensing period
The sensing period starts when the reset signal has returned from the high level to the low level. In other words, after the storage node has been reset in the reset period as mentioned above, the reset signal RS switches to the low level (−Vb), and thus the photodiode D1 becomes reverse biased. Then, the storage node changes to the −Vb direction due to a photocurrent that is in accordance with the amount of light that has been incident on the photodiode D1.
The above-described (A) readout period, (B) reset period, and (C) sensing period are considered to be one cycle, and sensor data is read out from the photosensor by repeatedly performing this cycle.
Citation List
Patent Literature
PTL 1: JP 2006-3857A
PTL 2: WO 2007/145346
PTL 3: WO 2007/145347